1. Field of the Invention
The present invention relates to digital watermarking of moving-picture data and in particular to a method and system for inserting watermark data into the moving-picture data.
2. Description of the Prior Art
With wide-spreading digital satellite broadcasting, Internet transmission and DVD (Digital Video Disk), etc. in recent years, digital images are becoming easily accessible to users. Since the quality of digital images does not deteriorate when they are copied, securing their copyrights has been an important issue.
For purposes of securing copyright protection for MPEG (moving picture experts group) data, there have been disclosed a watermarking system for embedding a plurality of watermarks into DCT (discrete cosine transform) coefficient domain of MPEG data and a watermark detecting system for extracting and detecting the embedded watermarks from the watermarked MPEG data (see Japanese Patent Application Unexamined Publication No. 11-55639).
More specifically, a plurality of watermarks different from each other are previously stored each corresponding to picture types (I-, B-, and P-pictures in MPEG). In the watermarking system, one watermark is selected from the plurality of watermarks depending on the type of a picture to be encoded and is embedded into the image data in units of 8×8 DCT coefficient block.
Assuming that f(1), f(2), . . . f(n) are DCT coefficients listed in frequency-ascending order, a watermarked coefficient F(i) is calculated by the following equation:F(i)=f(i)+α×avg(f(i))×w(i),where i=1, 2, 3, . . . , n, w(i) is an element of a watermark selected according to a normal distribution with mean mx=0 and variance σ2=1, α is a scaling element, and avg(f(i) )represents a local average over three DCT coefficients in the neighborhood, which is calculated by the following form:avg(f(i))=(|f(i−1)|+|f(i)|+|f(i+1)|)/3.
On the other hand, the watermark detecting system can detect an embedded watermark without the need of the original image data. The watermarked MPEG data is decoded and a watermark is extracted from the DCT components of the decoded data using the local average over the neighborhood. A watermark element W(i) is extracted by calculating the following form: F(i)/avg(F(i)). A watermark element W(i) is accumulated in one frame/field to produce WF(i).
A statistical similarity C between w(i) and WF(i) can be calculated using vector inner product as follows:C=WF×w/(|WF|×|w|),where WF=(WF(1), WF(2), . . . , WF(n)) and w=(w(1), w(2), . . . , w(n)).
If the statistical similarity C calculated as described above exceeds a predetermined threshold, it is determined that the corresponding watermark is embedded in the MPEG data.
However, the inventor found that a degree of embedded watermark effect on MPEG data is varied depending on the type of a picture in MPEG. If watermark frequency coefficients having a certain amplitude (level) is uniformly embedded to MPEG data regardless of the type of a picture as the prior art, then there are cases where the quality of image is deteriorated and the embedded watermark is hard to be detected.
More specifically, when the watermark is strongly embedded to the MPEG data, ease of watermark detection is enhanced but the quality of image is deteriorated. Contrarily, when the watermark is lightly embedded to the MPEG data, the quality of image is kept sufficiently but the embedded watermark becomes hard to be detected. In other words, the quality of a watermarked image is traded off against watermark detection efficiency.